In cold climates the Cold Filter Plugging Point (CFPP) of diesel fuels is very important and is specified in various standards such as the EN590 diesel fuel specification where the climate related requirements varies from −20° C. CFPP for countries such as Germany to −10° C. and −5° C. for countries such as Portugal and Greece. CFPP, measured according to the IP 309 and CEN EN116 test methods, is widely accepted for predicting low-temperature performance. It is the highest temperature at which a given volume of fuel fails to pass through a standardized filtration device in a specified time when cooled under standardized conditions.
The CFPP of FT derived diesel can be improved either by reducing the T95 cut point of the diesel or with the use of an additive. Alternatively, the CFPP of the GTL diesel can be improved by changing the process operating conditions or through a combination of the above options. To obtain a “winter” grade FT derived diesel conforming to a grade F CFPP specification of −20° C. with fixed process conditions and isocracking catalyst, the tail of the diesel needs to be fractionated lighter to remove the heavy waxy paraffinic hydrocarbons from the diesel pool. A consequence of such action includes yield losses.
CFPP improver additives work in two ways; by nucleation and by growth inhibition. These additives, are formulated in such a way that as the fuel cloud point is reached, it creates a large number of nuclei to which the first separating wax molecules attach themselves and form crystals. Growth inhibiting CFPP additives inhibit the normal development of wax crystals through adsorption onto the growing crystal surface, preventing plate-like growth. The resulting crystals have a more compact shape and will be less prone to agglomerate. Large numbers of small crystals form as a result and can pass more easily through filter screens that enable the engine to keep running.
One of the most important fuel characteristics influencing cold flow additive response is its wax content which will depends on the distillation profile of the fuel. Complex refineries with many downstream cracking units results in diesel with a narrow boiling range and are more difficult to treat.
The cold flow properties of diesel can either be improved by cutting lighter and remove the heavy tail of the diesel or it can be improved with a CFPP improver. It is known that removing the tail of the diesel will result in a decreased effect of a cold flow improver additive on its CFPP with a decrease in wax content of the fuel.
Surprisingly an approach has been found which overcomes the above known problem for improving the CFPP additive response of FT derived diesel.